Method and apparatus for separating the constituents of gaseous mixtures



. c. c, VAN NUYs 1,959,883 METHOD AND APPARATUS FOR SEPARATING THE CONSTTUENTS OF GASEOUS MIXTURES Filed July 1.6, 1932 May 22,1934.

lNVlzNTOR C/aqde C Van /Vuys BY Y www? ATTORNE 'Patented May 22, 1934 PATENT, OFFICE METHOD AND APPARATUS FOR SEPARAT- ING THE GONSTITUENTS OFl GASEOUS MIXTURES Claude C. van Nuys, Cranford, N. J., assigner to Air Reduction 6Claims.

This invention relates to the liquefaction and separation of the constituents of gaseous mixtures and is particularly applicable to the recovery of oxygen and nitrogen from the atmosphere although the principles hereinafter described may be employed in' separating the constituents of gaseous mixtures other than air.

The process forming the subject-matter of the invention is particularly of importance in con- 10 nection with large scale oxygen production for use in metallurgical processes. In such elds the oxygen produced is not required to be of a purity comparable with that produced in any of the well-known processes for air separation, but a more important consideration for a large-scale oxygen plant is the reductionof power consumption to the lowest possible ilgure. In fact in almost all manufacturing processes where oxygen in large quantities can be utilized, a purity of 95% or better is satisfactory.

,In the well known Claude process for producing oxygen of high purity, e. g., 9 91/2%, the separation of the constituents of air is accomplished by compression and coolingof the mixtureiolf-v lowed by a liquefaction in two -fractions at a pressure "somewhat below the initial pressure. TheJ first liquid fraction is obtainedby selectivev condensation in a tubular condenser employing the principle'of backward return so as to obtain a liquid enriched in oxygen to which is added sufcient liquid air obtained from a separate condenser called the liqueer to make up for losses dueto heat leakage.

The secondfraction consistingsubstantially of the more volatile constituent. nitrogen, delivered at the top of the backward return tubular condenser is liquefied by indirect contact with liquids accumulated at or near the bottom of the rectification column, and "delivered to the rectier at the top level, whereas the liquid constituting the first fraction enters the rectifier-at an intermediate level. This second liquid fracf tion, known as .the redux nitrogen \washes the oxygen from the ascending vapors by reciprocal evaporation and condensation and this permits separation of the constituents ln a substantially pure condition.

Let u's assume that the tubularfvaporizer in which the selective condensation takes place is suificiently eilicient so that the unliquelled gas leaving the top of the tubes is composed s ubstanf tially of the more volatile' constituent. nitrogen, while the enrichedliquidpbtaiped at the bottom of the condenser. is a liquid ofmaximum possible degree of enrichment in oxygen, that is. it is that other words, when the enriched liquid reaching Company, Incorporated, New York, N. Y., a corporation of New York Application .my 16, 1932, serial No. 622,916

(Cl. s2-175.5)

liquid which'will be in phase equilibrium with the Rossum air entering the bottom of the condenser. Under lthese conditions, if they could be realized, it is clear that the amount of unliqueiied nitrogen leaving the top of the condenser would be a 6o maximum. If the composition in oxygen of the enriched liquid constituting the lfirst fraction is less than the limit composition just specied, then the amount of the second fraction, that is,

the unliqueed nitrogen leaving the top of the tubular vaporizer will be less.

In the normal operation of this method, the attempt is generally made to produce only that amount of nitrogen leaving the top of the vaporizer and constituting ultimately the second liquid 7 0 fraction, which is necessary to deprive the vapors ascending at the level in the rectiication column at which vthe rst liquid fraction is delivered thereto, of the oxygen contained in those vapors. The degree of enrichment of the first fraction 75 thus produced is much less than that for a liquid in phase equilibrium with unseparated air.

This being the case, it is clear that the method fails to take advantage of the maximum possible degree of oxygen enrichment attainable in ac- '80 cordance with the principle of condensation by I backward return. On the contrary, this enrichment is carried on only to that point where the amount of reux nitrogen is most suitable for the requirements oi' the subsequent rectification. 85 This` feature ofthe well known Claude process just described constitutes an ineillciency, the result of -which is` to increase very materially the cost of 'the oxygen produced by the method over that .attainable by the improved method forming the subject-matter of this application.

It is the object of the present invention to provide a cycle whereby oxygen is produced in a very elcient manner on a large scale by removing, among others, the defect -just pointed out in connection with the well known Claude cycle. It is possible to add, in th saturated vapor condition, a substantial quantity'of unseparated low pressure air at an intermediate level of the upper rectier,v pro ed the greatest possible use is made of the' principle of selective liquefaction, that is, provided the greatest passible amolmt pi.' uncondensed nitrogen is removed from thetop of a the first l`-tnbular vaporizer or condenser, or vin the' b`owl or pot below the tubular condenser is as-nearly as possible enriched to the degree for phaseequilibrium with unseparated air.

' The u nseparated air, deli ered at the interme' di'le level oithe upper rec er, does not require any compression and it is. an object of the present invention to make the quantity of air there added as great as possible.

It will be understood that all pressures herein referred to are absolute pressures, and the expressions indirect contact and thermal contact refer to heat' exchanging relation, such as found in surface condensers, in which the fluids are separated by the walls of the tubes and exchange heating by conduction through these walls. l

Another object of the invention is to reduce the pressure of the air entering the lower end of the tubular condenser to the lowest possible figure, this pressure being determined by the necessity of the air ascending in the tubes to be partially liquefied by the liquid oxygen, more or less pure, surrounding the tubes of said tubular condenser. In the Claude process, as above described, the unf condensed nitrogen leaving the top of the tubular condenser is subsequently liquefied in a nonselective condenser by indirect contact with liquids accumulated at the bottom or at an intermediate level of the rectication colu The liquid or reflux nitrogen thus-obtained i admitted to the recticatlon column at the t p level. `In theClaude process this reiiux liquid nitrogen should be as pure as possible, thatis, free from oxygen, in order to prevent serious oxygen losses occurring at the top of the rectification column.

" Hence inthe Claude process the temperature of the residual'gas leaving the top of the rst tubular condenser is the temperature of saturation for substantially pure nitrogen at'the pressure existing in the tubesof said tubular condenser. The condensation in the tubes of the condenser is produced by indirect contact with the liquid oxy- /g'en accumulating at the bottom of the main rectier and around the tubes, and thus the pressure necessary in the tubes is that' which will cause pure nitrogen to condense in thermal contact with relatively pure oxygen at' substantially atmospheric pressure.

As a further feature of the present invention, I have discovered that, by reason of new and novel features about to be described, it is possible to reduce the'pressure necessary for condensation of the ascending vapors in the tubes of the tubular condenser above described. This is accomplished by making the second condenser, that is, the condenser in the Claude process in which the reux nitrogen is liquefied, also to operate as a selective backward return condenser. ThisA condenser constitutes the second tubular condenser in the apparatus in which myprocess is carried out, andis located at a higher level in the rectification column. The nitrogen which, in my process, neednot be pure, leavingthe top of the r'st tubular condenser or--vaporizer enters the bottom of the tubes of the second condenser just described and is partially liquefied therein, the liquid formed draining backward to the bot- 'l tom of the tubes while an uncondensed residue of high purity nitrogen leaves the top of this second condenser.

The liquid selectively condensed in the second tubular condenser is delivered to an upper level ofthe main rectier above the entrance of the enriched liquid produced at the bottom of the first tubular condenser. The level of admission of this' second entering liquid is not, however, at the top of the rectifier. The uncondensed nitrogen vapor leaving the top -of the second tubular condenser is next passed through a nonselective third condenser, being therein liqueed non-selectively, by 'means about to be de' scribed, and then delivered as reflux liquid nitrogen to the top level of the main rectifier. The condensation of this latter nitrogen I accomplish by indirect contact with a quantity of liquid oxygen in amount equivalent to the oxygen product of the process, thisliquid olwgen evaporating under subatmospheric pressure around the tubes of the third condenser just described. The degree of vacuum required for this condensation is relatively small but the result is to materially reduce thenecessary pressure of the incoming air, since in the rst tubular vaporizer or condenser it is no longer necessary to condense pure nitrogen by indirect contact with more or less pure oxygen at a pressure slightly above atmospheric.

In order that the liquids descending in the rectier and coming in contact with the upper part of the main or first tubular condenser shall be as cold as possible, in the process forming the subject-matter ofthe present application, I continue the trays of the main rectification column downwardly around the tubes of the first tubular condenser so that the liquid surrounding the upper part of the tubes is colder than the nue oxygen liquid reaching the bottom of said u es.-

By reason of the novel features just described, my process reduces the necessary pressure of the air entering the first or main tubular vaporizer to a very low figure and at the same time makes it possible to add a substantially greater amount of low pressure unseparated air at an intermediate level of the upper .or main rectifier. This latter result is due to two facts: First, at the lower pressure which I attain in the tubes of the rst tubular condenser, the degree of enrichment attainable in the oxygen-rich liquid accumulating at the bottom of the condenseris increased since it is well knowgi that the lower the pressure, the higher in oxygen percentage will be the liquid for phase equilibrium with unseparated air. Second, by reason of the fact that there are three liquid feeds or streams entering the main rectifier, the rectication throughout the 'main rectifier ismore eilicientin that the quantity of liquid entering at any one level in my process is less than the liquid entering at any one level of the rectifier of the Claude process where there are only two entering liquids. The reduction in the quantity of liquid entering any one level of a. rectier makes the rectication more favorable because at such a liquid entrance level the liquid descending and the vapor ascending need not be as close to phase 13( equilibriumwith each other and 'thus the interchange of the two constituents between the liquid and vapor is more rapid. I

The preferred type of apparatus is diagrammatically illustrated in the accompanying draw- 1w ing. The details of the apparatus,- such as are` well known to those skilled in the art, are omitted for the purposeof clarity. It is to be understood, moreover, that the invention is applicable in a variety of types of apparatus and in various modiiicationsof the known methods of separating the constituents of gaseous mixtures;

Referring to the drawing, the gaseous mixture to be separated, such as air, enters the apparatus through a pipe 1 and into a blower 2, and thence to a pipe 3 having branches 4 and 5. A miner portion of the air passes through the pipe 5 to a compressor '6 where it is compressed and discharged through a pipe 'I to a liqueer 8. A portion of the air leaves the liqueiier at an interl5 asV 8 through a pipe 11.

mediate levelthrough pipe 9 and enters an expansionmachine 10, where it is expanded with the performance of external work. The air leaves the expansion machine and re-enters the liqueer After passing counter-current through the liquer it ilows through a pipe 12 and joins another portion of air from the blower. 'I'he other portion of air continues on through the liqueer 8 where it is condensed and leaves as liquid through a pipe 13 to be delivered, together with the cold air from exchanger 31, to the bottom of the rectication column 14.

'I'he portion of the air which passes through pipe 4 is divided, part passing through a pipe 15 and the 'rest through a pipe 16. From pipe 16 the air entersa compressor 17 wherein it is compressed to that pressure necessary for condensation in the tubes of the irst tubular condenser hereinafter described. The compressed air leaves the compressor through a pipe 18, is again divided, one portion passing through a pipe 19 and valve 19a and another portion passing through a pipe 20 and valve 21. VThis latter portion enters 'a heat exchanger 22 in which it is cooled by the already-separated oxygenl vapor. This cooled compressed portion of air leaves the exchanger 22 through a pipe 23 and enters the vbottom of the rectification column 14 through a pipe 24.

That portion of the air entering pipe 15 passes through valve 25 and pipe 26 and after the addition of the air from pipe 12 enters a 'heat exchanger 27, where it is cooled by a portion of the gaseous nitrogen product. This air leaves the heat exchanger through a pipe 28 and enters a heat exchanger 29 to be further cooled bythe whole of the nitrogen product comingdirectly from therectication column 14. From the exchanger 29 this .air passes through a pipe 30 and enters the main rectification column at an intermediate level.

That portion of air from the compressor 17 which enters the pipe 19 is cooled in a heat exchanger 31 by a portion of gaseous nitrogen product as hereinafter described. This cooled portion of air passes through a pipe 32, to a pipe 33 where it is joined by the liquid air in the pipe 13. The cooled air and liquid air flow through pipe 33, where they join the portion of cooled compressed air from exchanger 22, and thence through pipe 24 intol the bottom of the rectication column.

The rectification column 14 is divided by a partition 34 into a compartment 35 communicating with a tubular condenser 36. 'I'he cold compressed air entering compartment 35 is partially liquefied in the tubular vaporizer 36 by liquid ox-v ygen surrounding the tubes. 'I'he vapor rising through the tubes 37 of the vaporizer 36 leaves the top thereof through a pipe 38 and passes into the bottm of a second condenser 39. In rising through the tubes 40 of the 'backward retum condenser 39, the gaseous mixture is subjected to selective liquefaction and consequent separation of the less volatile constituent, oxygen. The liquid runs backward through the tubes 40 in contact with the entering gaseous mixture to effect a further separation in accordance with the well known principles of backward return condensation. The accumulated liquid, which will be more or less impure nitrogen and may containy 15%, more or less, of oxygen, passes through ay pipe 41 and pressure-reducing valve 42 to the upper part ot the main rectiier at a level well above the entrance of the enriched liquid produced at the bottom of the 1li-st tubular concondenser 44. The nitrogen which condenses at the bottom of this condenser 44 is led through pipe 46 and pressure-reducing valve 47 to the top of the main rectifier, i. e., above the entering level of the reux liquid from the second condenser 39. The liquid flows downwardly over the group of trays 48, 49 and 50 and is subjected to rectification with the resultant separation of the more volatile constituent, nitrogen, and the nal accumulation of a liquid consisting of from about upward to 99.5% of the less volatile constituent, oxygen, at the bottom of the compartment 51 where the oxygen surrounds the tubes37.. Oxygen of a purity of 99.5% is suitable for the present oxygen business where oxygen is used as a welding or cutting gas, whereas a purity of around gaseous oxygen leaves thethird condenser 44 through a pipev54, `passes through the heat exchanger 22 where it cools a portion of the unseparated compressed air, and leaves the apparatus' through a pipe 55, vacuum pump 56 and pipe 57.

'I'he liquid which collects in the bottom of compartment 35, as a result ofthe selective condensation in the tubular vaporizer 36, contains a percentage of oxygen as closely equal as possible to that of the liquid for phaseA equilibrium with the air entering pipe 24. 'I'his enriched liquid is drawn off through pipe 58 and enters a heat exchanger 59 in which it is cooled by the cold nitrogen product from the exchanger 29. The en- -riched liquid then leaves exchanger 59- through pipe 69 and pressure-reducing valve 61 to enter the main rectification column at an intermediate level, above that at which the additional uncompressed air enters and below that at which the lower reilux is added. The nitrogen product is taken. from the top of the rectification column through a pipe 62 and passes rst through the exchanger 29, where it cools the low pressure incoming air coming through pipe 28 and then through pipe 63 and exchanger 59, where it cools the enriched oxygen from the bottom of the column. The nitrogen leaves exchanger 59 through a pipe 64 and is divided into two parts. One part passes through a pipe 65 to the exchanger 31, where it cools a portion of the incoming compressed air, and then leaves the apparatus through a pipe 66 and valve 67. The other por- -tion enters exchanger 27 through a pipe 68 and les tering and leaving the main rectier ift. The required refrigerative effect is thus produced in a highly efficient and convenient manner.

The invention can be used with other gaseous mixtures than air. The method can be carried out with apparatus different from that illustrated.

backward return' by thermal contact with llquefied products evaporating at substantially atmospheric pressure,'subjecting the unliqueed residue of the gaseous mixture to selective liquefaction at a-lower temperature with 'backward return by thermal contact with liqueed products evaporating. at substantially atmospheric press ure, subjecting the still unliquefied residue of the gaseous mixture to non-selective liquefaction at a still lower temperature by thermal contact with a liquid product of the separationevaporating at sub-atmospheric pressure, thereby producing three liquids of different composition, that of the final liquefaction being substantially the constitvuent of lowest boiling point, and utilizing the three liquids at different levels to rectify vapors from previously introduced portions of such liquids evaporating at substantially atmospheric-- pressure. Y

2. The method of separating the constituents of gaseous mixtures which comprises subjecting the gaseous mixtures to selective liquefaction with backward return by thermal contact with liquefied products evaporating at substantially atmospheric pressure, subjecting the unliqueed residue of the gaseous mixture to selective liquefaction at a lower temperature with backward return by thermal contact with liquefied products evaporating at substantially .atmospheric pressure, subjecting the still unliquefled residue of the gaseous mixture to non-selective li uefaction at a still lower temperature bythe mal contact with a liquid .product of the separation evaporating at sub-atmospheric pressure, thereby producing three liquids of different composition, that of the final liquefaction being substantially the constituent of lowest boiling point, utilizing the three liquids at different levels to rectify vapors from previously introduced portions of such liquids evaporating at substantially atmospheric pressure, and introducing at' an intermediate level of the rectification a quantity of cold but previously unlique'fed air including the initial constituents thereof.

3. The method of separating oxygen from air ,which comprises subjecting the' air to selective liquefaction with backward return by thermal contact with liquefied products evaporating at substantially atmospheric pressure to obtain a yliquid, rich in oxygen, withdrawing vapor from said selective liquefaction in such quantities that a portion of the oxygen of the airpasses therev from along with the nitrogen, subjecting the unliquefied oxygen and nitrogen of said selective liquefaction to a second selective liquefaction at a lower temperature with backward return by thermal contact with liquefied products evaporating at substantially atmospheric pressure to obtain another oxygen-containing liquid, withdrawing substantially pure nitrogen from 4the zone of the second selectivev liquefaction,

subjecting said substantially pure nitrogen to non-selective liquefaction at a still lower temperature by thermal contact with liquid oxygen of the separation evaporating at sub-atmospheric pressure. and utilizing the two oxygen-containing liquids and the liquid nitrogen at different levels to rectify vapors from previously introduced portions of such liquids evaporating at substantially atmospheric pressure./

4. The method of separating oxygen which comprises subjecting the air to selective liquefaction with backward return by thermal` cont'act with liquefied products'evaporating at substantially atmospheric pressure to obtain a liquid rich in oxygen, withdrawing vapor from said selective liquefaction in such quantities that a portion qf the oxygen of the air passes therefrom along with the nitrogen, subjecting the unliquefied loxygen and nitrogen of said selective liquefaction to a second selective liquefaction at a lower temperature with backward return by thermal contact with liquefied products evaporating at substantially atmospheric pressure to obtain another oxygen-containing liquid, withdrawing substantially pure nitrogen from the zone of the second selective liquefaction, subjecting said substantially pure nitrogen to non-selective liquefaction ata still lower temperature by thermal contact with liquid oxygen of-the separation evaporating at sub-atmospheric pressure, and utilizing the two oxygen-containing liquids and the liquid nitrogen at diiferent levels to rectify vapors from previouslyintroduced portions of such liquids evaporating at substantially atmospheric pressure with the liquid most enriched in oxygen being utilized at the lowest level and the liquefied nitrogen being utilized at the uppermost level.

5. The method of separating oxygen from air which comprises supplying air to aaliquefaction zone at a pressure below that at which high purity nitrogen will condense in thermal contact with relatively pure liquid oxygen at substantially atmospheric pressure and then subjecting the air to selective liquefaction with backward return by thermal contact with liquefied products sure to obtain a'liquid rich in oxygen, subjectfrom air ing the unliqueiied residue of the air to selective I liquefaction at a lower temperature with backward return by vthermal contact with liquid products evaporating at substantially atmospheric pressure to obtain oxygen-containing liquid, subjecting the still unliqueed residue of the air t'o non-selective liquefaction-at a still lower temperature by f thermal contact with liquid oxygen of the separation evaporating at sub-atmospheric pressure to obtain substantially pure liquid nitrogen, utilizing the three liquids at different levels to-liquefy vapors from previously introduced portions of such liquids evaporating at substantially atmospheric pressure, and introducing at an intermediate level of the rectification va quantity of cold but previously unliquefied air including the initial constituents thereof.

6. Apparatus for the separation .of oxygen from air, comprising a rectification column, a backward return condenser at the bottom of the rectification column and in contact with the liquid Yoxygen product of the rectification column, a second backward return condenser in' an intermediate level of the rectification column in position to be cooled by the products at that level of the column, a conduit through which gases escaping from the top of the rst condenser'ento the nitrogen condenser and into thermal contact with the nitrogen, and a vacuum pump con- Y nected with the nitrogen condenser for withdrawing oxygen vapor from that condenser so thatv the pressure on the oxygen is sub-atmospheric and the oxygen boils to refrigerate and condense the nitrogen.

CLAUDE C. VAN NUYS. 

